The present invention relates to edible microcapsules suitable for dried powders and foods containing the same.
Edible microcapsules containing a hydrophobic core substance, such as fat-soluble vitamins (e.g., vitamin A, vitamin D and the like), flavor oils, fats and oils or fatty acids (e.g., eicosapentaenoic acid (hereinafter referred to as xe2x80x9cEPAxe2x80x9d), docosahexaenoic acid (hereinafter referred to as xe2x80x9cDHAxe2x80x9d) and the like) and the like, are essentially required to be in the form of dried powder from the standpoint of preservation stability.
The state-of-the-art microcapsules having a small particle size of not larger than 100 xcexcm are obtained by spray drying with a spray dry apparatus. This method has been utilized, in general, for granulating a liquid composition into fine particles. When microcapsules are prepared according to this method, a dispersing solution in which a hydrophobic core substance, such as a flavor oil, a fatty acid or the like, is emulsified in an aqueous solution of a coating substance, such as gum arabic, dextrin or the like, is prepared, and the obtained dispersing solution is spray-dried in a high-temperature air stream to obtain powder particles of microcapsules. This method is characterized in that the dispersing solution is made into fine droplets having a large total surface area by spray drying into a high-temperature air stream, thereby assuring an increased drying effect. Accordingly, the method is rather suitable to prepare powder particles having a particle size of not larger than 100 xcexcm, preferably the order of several tens of microns.
For preparing large particles having a particle size of not smaller than 1,000 xcexcm, on the other hand, a so-called orifice method using a double orifice (with a structure providing an inner orifice within an outer orifice of relatively larger calibre) has been put to practical use, in which a liquefied capsule wall-forming substance are fed under pressure into the outer orifice and then extruded therefrom hugging its calibre, wraps around a droplet of the core substance are dripped out of a nozzle at the outlet of the inner orifice, and the thus wrapped up droplets are dropped into an oil bath for cooling and hardening (solidifying) the capsule wall, and the thus made microcapsules are collected from the oil and further dried.
Apart from the above-described two methods that have been in practical use, coacervation has been attracting attention as a technique for obtaining edible microcapsules having a particle size of about 100 to 400 xcexcm, which size is the most suitable for addition to foods. Coacervation is divided into two methods, i.e., complex coacervation and simple coacervation. Salting-out is included in simple coacervation. Complex coacervation consists in coacervation (phase separation) induced by the electrolytical action between polycations and polyanions, which causes separation of a polymer rich phase out of the equilibrium liquid with subsequent precipitation and adsorption around surface of the droplets of a hydrophobic core substance to form a capsule wall. Examples of edible combinations of polycations and polyanions include gelatin-gum Arabic, gelatin-CMC Na, and gelatin-carrageenan.
The outline of encapsulation by complex coacervation is as follows. All of the following processes are carried out under continuous agitation.
1) A hydrophobic core substance is emulsified or dispersed in a positively charged water-soluble polymer solution having the properties of a protective colloid.
2) An oppositely charged hydrophilic colloid solution is added thereto.
3) The colloid concentration, pH, temperature and the like of the system are controlled so as to induce coacervation (phase separation), thereby to precipitate a colloid rich phase of the water-soluble polymer onto the surface of the hydrophobic core substance to form a microcapsule wall.
4) The capsule wall of the resulting microcapsules is insolublized and stabilized by crosslinking.
In the step of insolublizing the capsule wall, aldehydes, such as formaldehyde, glutaraldehyde and the like, are generally used as a crosslinking agent for hardening (in the present invention, often referred to as a xe2x80x9ccrosslinking insolublizing agentxe2x80x9d); however, they are not suitable for foods. Tannic acid, gallic acid and the like are known as a crosslinking agent for hardening applicable to foods; however, the crosslinking effect thereof is not sufficient. Moreover, some crosslinking agents for hardening usable for foods give their odor or taste to the capsule wall made of, e.g., gelatin.
Under these circumstances, the crosslinking treatment for hardening or solidifying (in the present invention, often referred to as xe2x80x9ccrosslinking insolublizing treatmentxe2x80x9d) using transglutaminase, which is an enzyme disclosed in JP-A-1-27471, is remarkably superior to the above-described conventional crosslinking treatment for hardening or solidifying (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d). Application of transglutaminase as a crosslinking agent for hardening or solidifying of a capsule wall to edible microcapsules obtained by coacervation has been already reported (see JP-A-5-292899).
Spray drying using a spray dry apparatus is a very old technique that has been in large practical use. It has been utilized in powdering skim milk since the early twentieth century (Nihon Funtai Kogyo Gijyutu Kyokai (ed.), Granulating Handbook).
With the broadening of application, the spray dry apparatus has increased in speed and automation, and the technique has been in wide use as a means for finely granulating a liquid substance.
Spray drying has the following characteristics: (1) powder particles can be obtained directly from a liquid substance; (2) a liquid substance is made into fine droplets having a large surface area by the jet of a liquid substance from a pressure nozzle or the centrifugal force of a high-speed rotary disc, thereby to increase the drying efficiency; and (3) spray drying is suitable for continuous mass production. Therefore, spray drying is a very reasonable technique for finely granulating a liquid substance.
However, dry-powdered products obtained by spray drying comprise many hollow spherical particles. As is understood from the attributes of the process, it is difficult to obtain dried powder particles of microcapsules in which individual particles of a core substance are completely enveloped (encapsulated) with a continuous wall even if a suspension in which the core substance is emulsified in the encapsulating material solution is dry-granulated. Furthermore, if the capsule wall obtained by spray drying is required to have some protecting properties, the wall material should be used in a proportion of 70 to 80% based on the microcapsules, which means that the amount of the core substance is very small.
In the orifice method using a double orifice (with a structure providing an inner orifice within an outer orifice of relatively larger calibre), a microcapsule is obtained in such that a liquid wall forming substance, such as gelatin and the like, is extruded from the outer orifice, wraps around a droplet of the liquid core substance are dripped from nozzle of the inner orifice, and the wrapped up particles are dipped into an oil bath for cooling and solidifying the wall forming substance. The microcapsules obtained by this method with a subsequent process for drying are far more satisfactory than those obtained by spray drying in that the core substance is completely wrapped up with a continuous wall.
However, because the orifice method requires a mechanical process of feeding, extruding and dripping a liquid core substance and a liquid capsule wall substance as well to an extremely narrow calibre of the orifices with a high pressure pump, it is very difficult to obtain fine particles having a particle size of not larger than 1,000 xcexcm and having a thin capsule wall. Mass production by the orifice method can be achieved only by increasing the number of double orifice and increasing the speed of such mechanical operations as feeding, extruding and dripping with a fine control of eventual size of a droplet come out of the orifice. This requires expensive equipment for increasing the pump pressure, and automated steps to optimize amount of the core substance and the capsule wall substance to be fed, extruded and dripped and for subsequent cooling, drying, recovering and defatting, and integrating subtle control into the automated steps.
The complex coacervation for obtaining powder particles of edible microcapsules having a particle size of 100 to 400 xcexcm, which size gives no discomfort in the mouth, has previously been proposed in JP-A-5-292899. According to the disclosed method, the final step of drying a microcapsule slurry prepared by complex coacervation is carried out by spray drying with a spray dry apparatus for simultaneously removing water and drying in the case of obtaining edible microcapsules having a small particle size of not larger than 100 xcexcm.
Also, for preparation of edible microcapsules having a medium particle size of not smaller than 100 xcexcm, the step of drying by a fluidized bed method or blowing of warm or hot air is carried out after removing water of the microcapsule slurry through filtrations to obtain filter cake and dusting them with fine particle of starch powder to prevent the primary microcapsule particles from being agglomerated while drying.
Since the capsule wall of the microcapsules prepared by complex coacervation are highly swollen with water (having a high water content), the efficiency of dehydration before drying treatment is poor. Besides, the surface of the capsule wall is so sticky that a large quantity of an antiblock agent is required, but yet microcapsules in the primary particle state are hardly obtained, showing tendency to agglomeration, and the resulting product has a reduced content of the core substance.
An object of the present invention is to provide edible microcapsules which (1) have a capsule wall having a small degree of swelling (low water content), (2) are suited to dried powder because of its low stickiness being negligible, and (3) have a reduced capsule wall thickness to ensure a high content of the core substance.
Another object of the present invention is to provide foods containing the edible microcapsules.
These and other objects of the present invention have been attained by an edible microcapsule comprising a core and a capsule wall, wherein said core is an edible hydrophobic substance; said capsule wall is formed by salting-out of a combination of a protein and an edible salt; and transglutaminase is used as a crosslinking agent for hardening (solidifying) said capsule wall.
Furthermore, these and other objects of the present invention have been attained by a food comprising the above-described edible microcapsule.
Moreover, these and other objects of the present invention have been attained by a method for preparing the above-described edible microcapsule.